Previous methods for biasing complementary Darlington stages incorporated either resistors or extra constant current sources to provide operating currents for the Darlington transistors. Two common methods are resistor biasing, shown in FIGS. 1 and 2, and constant current source biasing, shown in FIG. 3.
One method of accomplishing resistor biasing is shown in FIG. 1. A resistor R1 is connected between the emitters of the first Darlington transistors QN4 and QP4. Since the Vbe's of the two output Darlington transistors QN5 and QP5 will appear across resistor R1, resistor R1 must be made large for low current operation, since the voltage drop is approximately 1.4 volts at 27.degree. C. The value for resistor R1 would typically be approximately 140K ohms for 10 microamps of current through transistors QN4 and QP4.
Resistors of that size require large areas of silicon to implement. Since most diffused or implanted resistors have positive temperature coefficients while those of the Vbe's of bipolar transistors are negative, large variations in the biasing current of the first Darlington transistors QN4 and QP4 will occur over temperature. Variations in the sheet resistance value of the process will also cause variations in the biasing current.
A second method of resistor biasing is illustrated in FIG. 2. Resistors R1 and R2 are connected across the base and emitters of Darlington transistors QN5 and QP5, respectively. In this case, the Vbe of QN5 will appear across resistor R1, while the Vbe of QP5 will appear across resistor R2. Since the Vbe of transistors QN5 and QP5 are approximately 0.7 volts at 27.degree. C., resistors R1 and R2 will have to be made large for low current operations. The typical value for resistors R1 and R2 would be approximately 70K ohms for 10 microamps of current through QN4 and QP4. Again, resistors of this size would require large areas of silicon to implement. Furthermore, since most diffused or implanted resistors have positive temperature coefficients while those of the Vbe's of bipolar transistors are negative, large variations will again occur in the biasing current of the first Darlington transistors QN4 and QP4 with variations in temperature. Variations in the sheet resistance value of the process also cause variations in the biasing current.
Another method of biasing is shown in FIG. 3. As shown in FIG. 3, two constant current sources I1 and I2 are used to bias the first Darlington transistors QN4' and QP4' respectively. This method of biasing requires one current source connected to each supply rail. Although the current supplied by current sources I1 and I2 will remain more constant than that provided by the resistor bias described above, the tracking of these currents to the main bias current in the output stage (I) with variations in supply voltage and temperature must also be considered. The current sources I1 and I2 will need to have cascoded outputs to minimize the current variation with output voltage swing if the early voltage of the current source transistors is low. Since one reference current generator is usually used to establish bias current for the Darlington stage, a current mirror connected to the other supply rail will have to be added to generate the opposite polarity current for the complementary first Darlington device. Additional mismatch among the various components must also be taken into account in the design.